How microstructures, oxide layers, and charge transfer reactions influence double layer capacitances. Part 1: impedance spectroscopy and cyclic voltammetry to estimate electrochemically active surface areas (ECSAs).

Varying the electrode potential rearranges the charges in the double layer (DL) of an electrochemical interface by a resistive-capacitive current response. The capacitances of such charge relocations are frequently used in the research community to estimate electrochemical active surface areas (ECSAs), yet the reliability of this methodology is insufficiently examined. Here, the relation of capacitances and ECSAs is critically assessed with electrochemical impedance spectroscopy (EIS) and cyclic voltammetry (CV) data on polished (Au, Ti, Ru, Pt, Ni, glassy carbon, graphite plate) and porous (carbon fleeces) electrodes. By investigating this variety of electrodes, the frequency-dependencies observed in the measured capacitances are shown to arise from the inherent resistive-capacitive DL response, charge transfer reactions, and resistively damped capacitive currents in microstructures (such as pores, pinholes, or cracks). These frequency-dependencies are typically overlooked when capacitances are related to ECSAs. The capacitance at the specimen-characteristic relaxation frequency of the resistive-capacitive DL response is proposed as a standardized capacitance-metric to estimate ECSAs. In 1 M perchloric acid, the polished gold electrode and the high-surface area carbon fleeces show ratios of capacitance-metric over surface-area of around 3.7 µF cm-2. Resistively damped currents in microstructures and low-conducting oxide layers are shown to complicate trustworthy capacitance-based estimations of ECSAs. In the second part of this study, advanced equivalent circuits models to describe the measured EIS and CV responses are presented.

The electrocatalytic activity for the hydrogen evolution reaction on alloys is determined by element-specific adsorption sites rather than d-band properties.

Trends of the electrocatalytic activities for the hydrogen evolution reaction (HER) across transition metals are typically explained by d-band properties such as center or upper edge positions in relation to Fermi levels. Here, the universality of this relation is questioned for alloys, exemplified for the AuPt system which is examined with electrocatalytic measurements and density functional theory (DFT) calculations. At small overpotentials, linear combinations of the pure-metals' Tafel kinetics normalized to the alloy compositions are found to precisely resemble the measured HER activities. DFT calculations show almost neighbor-independent adsorption energies on Au and Pt surface-sites, respectively, as the adsorbed hydrogen influences the electron density mostly locally at the adsorption site itself. In contrast, the density of states of the d-band describe the delocalized conduction electrons in the alloys, which are unable to portray the local electronic environments at adsorption sites and related bonding strengths. The adsorption energies at element-specific surface sites are related to overpotential-dependent reaction mechanisms in a multidimensional reinterpretation of the volcano plot for alloys, which bridges the found inconsistencies between activity and bonding strength descriptors of the common electrocatalytic theory for alloys.

U(V) stabilization via aliovalent incorporation of Ln(III) into oxo-salt framework.

Pentavalent uranium compounds are key components of uranium's redox chemistry and play important roles in environmental transport. Despite this, well-characterized U(V) compounds are scarce primarily because of their instability with respect to disproportionation to U(IV) and U(VI). In this work, we provide an alternate route to incorporation of U(V) into a crystalline lattice where different oxidation states of uranium can be stabilized through the incorporation of secondary cations with different sizes and charges. We show that iriginite-based crystalline layers allow for systematically replacing U(VI) with U(V) through aliovalent substitution of 2+ alkaline-earth or 3+ rare-earth cations as dopant ions under high-temperature conditions, specifically Ca(UVIO2)W4O14 and Ln(UVO2)W4O14 (Ln = Nd, Sm, Eu, Gd, Yb). Evidence for the existence of U(V) and U(VI) is supported by single-crystal X-ray diffraction, high energy resolution X-ray absorption near edge structure, X-ray photoelectron spectroscopy, and optical absorption spectroscopy. In contrast with other reported U(V) materials, the U(V) single crystals obtained using this route are relatively large (several centimeters) and easily reproducible, and thus provide a substantial improvement in the facile synthesis and stabilization of U(V).

Toward Quantitative Electrodeposition via In Situ Liquid Phase Transmission Electron Microscopy: Studying Electroplated Zinc Using Basic Image Processing and 4D STEM.

High energy density electrochemical systems such as metal batteries suffer from uncontrollable dendrite growth on cycling, which can severely compromise battery safety and longevity. This originates from the thermodynamic preference of metal nucleation on electrode surfaces, where obtaining the crucial information on metal deposits in terms of crystal orientation, plated volume, and growth rate is very challenging. In situ liquid phase transmission electron microscopy (LPTEM) is a promising technique to visualize and understand electrodeposition processes, however a detailed quantification of which presents significant difficulties. Here by performing Zn electroplating and analyzing the data via basic image processing, this work not only sheds new light on the dendrite growth mechanism but also demonstrates a workflow showcasing how dendritic deposition can be visualized with volumetric and growth rate information. These results along with additionally corroborated 4D STEM analysis take steps to access information on the crystallographic orientation of the grown Zn nucleates and toward live quantification of in situ electrodeposition processes.

Machine Learning Isotropic g Values of Radical Polymers.

Methods for electronic structure computations, such as density functional theory (DFT), are routinely used for the calculation of spectroscopic parameters to establish and validate structure-parameter correlations. DFT calculations, however, are computationally expensive for large systems such as polymers. This work explores the machine learning (ML) of isotropic g values, giso, obtained from electron paramagnetic resonance (EPR) experiments of an organic radical polymer. An ML model based on regression trees is trained on DFT-calculated g values of poly(2,2,6,6-tetramethylpiperidinyloxy-4-yl methacrylate) (PTMA) polymer structures extracted from different time frames of a molecular dynamics trajectory. The DFT-derived g values, gisocalc, for different radical densities of PTMA, are compared against experimentally derived g values obtained from in operando EPR measurements of a PTMA-based organic radical battery. The ML-predicted giso values, gisopred, were compared with gisocalc to evaluate the performance of the model. Mean deviations of gisopred from gisocalc were found to be on the order of 0.0001. Furthermore, a performance evaluation on test structures from a separate MD trajectory indicated that the model is sensitive to the radical density and efficiently learns to predict giso values even for radical densities that were not part of the training data set. Since our trained model can reproduce the changes in giso along the MD trajectory and is sensitive to the extent of equilibration of the polymer structure, it is a promising alternative to computationally more expensive DFT methods, particularly for large systems that cannot be easily represented by a smaller model system.

Post-treatment strategies for pyrophoric KOH-activated carbon nanofibres.

The effect of two atmospheric post-treatment conditions directly after the KOH activation of polyacrylonitrile-based nanofibres is studied in this work. As post-treatment different N2 : O2 flow conditions, namely high O2-flow and low O2-flow, are applied and their impact on occurring reactions and carbon nanofibres' properties is studied by thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), scanning electron microscopy (SEM), Raman spectroscopy, elemental analysis and CO2 and Ar gas adsorption. At high O2-flow conditions a pyrophoric effect was observed on the KOH-activated carbon nanofibers. Based on the obtained results from the TGA and DSC the pyrophoric effect is attributed to the oxidation reactions of metallic potassium formed during the KOH activation process and a consequent carbon combustion reaction. Suppression of this pyrophoric effect is achieved using the low O2-flow conditions due to a lower heat formation of the potassium oxidation and the absence of carbon combustion. Compared to the high O2-flow samples no partial destruction of the carbon nanofibers is observed in the SEM images. The determination of the adsorption isotherms, the surface area, the pore size distribution and the isosteric enthalpies of adsorption show the superior properties under low O2-flow conditions. The present micropore volume is increased from 0.424 cm3 g-1 at high O2-flow to 0.806 cm3 g-1 for low O2-flow samples, resulting in an increase of CO2 adsorption capacity of 38% up to 6.6 mmol g-1 at 1 bar. This significant improvement clearly points out the importance of considering highly exothermic potassium oxidation reactions and possible post-treatment strategies when applying KOH activation to electrospun carbon nanofiber materials.

Ultrathin Two-dimensional Layered Composite Carbosilicates from in situ Unzipped Carbon Nanotubes and Exfoliated Bulk Silica.

A key task in today's inorganic synthetic chemistry is to develop effective reactions, routes, and associated techniques aiming to create new functional materials with specifically desired multilevel structures and properties. Herein, we report an ultrathin two-dimensional layered composite of graphene ribbon and silicate via a simple and scalable one-pot reaction, which leads to the creation of a novel carbon-metal-silicate hybrid family: carbosilicate. The graphene ribbon is in situ formed by unzipping carbon nanotubes, while the ultrathin silicate is in situ obtained from bulk silica or commercial glass; transition metals (Fe or Ni) oxidized by water act as bridging agent, covalently bonding the two structures. The unprecedented structure combines the superior properties of the silicate and the nanocarbon, which triggers some specific novel properties. All processes during synthesis are complementary to each other. The associated synergistic chemistry could stimulate the discovery of a large class of more interesting, functionalized structures and materials.

In operando NMR investigations of the aqueous electrolyte chemistry during electrolytic CO2 reduction.

The electrolytic reduction of CO2 in aqueous media promises a pathway for the utilization of the green house gas by converting it to base chemicals or building blocks thereof. However, the technology is currently not economically feasible, where one reason lies in insufficient reaction rates and selectivities. Current research of CO2 electrolysis is becoming aware of the importance of the local environment and reactions at the electrodes and their proximity, which can be only assessed under true catalytic conditions, i.e. by in operando techniques. In this work, multinuclear in operando NMR techniques were applied in order to investigate the evolution of the electrolyte chemistry during CO2 electrolysis. The CO2 electroreduction was performed in aqueous NaHCO3 or KHCO3 electrolytes at silver electrodes. Based on 13C and 23Na NMR studies at different magnetic fields, it was found that the dynamic equilibrium of the electrolyte salt in solution, existing as ion pairs and free ions, decelerates with increasingly negative potential. In turn, this equilibrium affects the resupply rate of CO2 to the electrolysis reaction from the electrolyte. Substantiated by relaxation measurements, a mechanism was proposed where stable ion pairs in solution catalyze the bicarbonate dehydration reaction, which may provide a new pathway for improving educt resupply during CO2 electrolysis.

Correction to "Bimetallic Copper-Silver Catalysts for the Electrochemical Reduction of CO2 to Ethanol".

[This corrects the article DOI: 10.1021/acsaem.3c00985.].

Impact of Electrostatic Interaction on Non-radiative Recombination Energy Losses in Organic Solar Cells Based on Asymmetric Acceptors.

Reducing non-radiative recombination energy loss (ΔE3 ) is one key to boosting the efficiency of organic solar cells. Although the recent studies have indicated that the Y-series asymmetric acceptors-based devices featured relatively low ΔE3 , the understanding of the energy loss mechanism derived from molecular structure change is still lagging behind. Herein, two asymmetric acceptors named BTP-Cl and BTP-2Cl with different terminals were synthesized to make a clear comparative study with the symmetric acceptor BTP-0Cl. Our results suggest that asymmetric acceptors exhibit a larger difference of electrostatic potential (ESP) in terminals and semi-molecular dipole moment, which contributes to form a stronger π-π interaction. Besides, the experimental and theoretical studies reveal that a lower ESP-induced intermolecular interaction can reduce the distribution of PM6 near the interface to enhance the built-in potential and decrease the charge transfer state ratio for asymmetric acceptors. Therefore, the devices achieve a higher exciton dissociation efficiency and lower ΔE3 . This work establishes a structure-performance relationship and provides a new perspective to understand the state-of-the-art asymmetric acceptors.

Li-Ion Conductivity of Single-Step Synthesized Glassy-Ceramic Li10GeP2S12 and Post-heated Highly Crystalline Li10GeP2S12.

Li10GeP2S12 is a phosphosulfide solid electrolyte that exhibits exceptionally high Li-ion conductivity, reaching a conductivity above 10-3 S cm-1 at room temperature, rivaling that of liquid electrolytes. Herein, a method to produce glassy-ceramic Li10GeP2S12 via a single-step utilizing high-energy ball milling was developed and systematically studied. During the high energy milling process, the precursors experience three different stages, namely, the 'Vitrification zone' where the precursors undergo homogenization and amorphization, 'Intermediary zone' where Li3PS4 and Li4GeS4 are formed, and the 'Product stage' where the desired glassy-ceramic Li10GeP2S12 is formed after 520 min of milling. At room temperature, the as-milled sample achieved a high ionic conductivity of 1.07 × 10-3 S cm-1. It was determined via quantitative phase analyses (QPA) of transmission X-ray diffraction results that the as-milled Li10GeP2S12 possessed a high degree of amorphization (44.4 wt %). To further improve the crystallinity and ionic conductivity of the Li10GeP2S12, heat treatment of the as-milled sample was carried out. The optimal heat-treated Li10GeP2S12 is almost fully crystalline and possesses a room temperature ionic conductivity of 3.27 × 10-3 S cm-1, an over 200% increase compared to the glassy-ceramic Li10GeP2S12. These findings help provide previously lacking insights into the controllable preparation of Li10GeP2S12 material.

Multimodal investigation of electronic transport in PTMA and its impact on organic radical battery performance.

Organic radical batteries (ORBs) represent a viable pathway to a more sustainable energy storage technology compared to conventional Li-ion batteries. For further materials and cell development towards competitive energy and power densities, a deeper understanding of electron transport and conductivity in organic radical polymer cathodes is required. Such electron transport is characterised by electron hopping processes, which depend on the presence of closely spaced hopping sites. Using a combination of electrochemical, electron paramagnetic resonance (EPR) spectroscopic, and theoretical molecular dynamics as well as density functional theory modelling techniques, we explored how compositional characteristics of cross-linked poly(2,2,6,6-tetramethyl-1-piperidinyloxy-4-yl methacrylate) (PTMA) polymers govern electron hopping and rationalise their impact on ORB performance. Electrochemistry and EPR spectroscopy not only show a correlation between capacity and the total number of radicals in an ORB using a PTMA cathode, but also indicates that the state-of-health degrades about twice as fast if the amount of radical is reduced by 15%. The presence of up to 3% free monomer radicals did not improve fast charging capabilities. Pulsed EPR indicated that these radicals readily dissolve into the electrolyte but a direct effect on battery degradation could not be shown. However, a qualitative impact cannot be excluded either. The work further illustrates that nitroxide units have a high affinity to the carbon black conductive additive, indicating the possibility of its participation in electron hopping. At the same time, the polymers attempt to adopt a compact conformation to increase radical-radical contact. Hence, a kinetic competition exists, which might gradually be altered towards a thermodynamically more stable configuration by repeated cycling, yet further investigations are required for its characterisation.

Laboratory X-ray computed tomography imaging protocol allowing the operando investigation of electrode material evolution in various environments.

A robust imaging protocol utilizing laboratory XCT is presented. Hybrid 2D/3D imaging at different scales with real-time monitoring allowed to assess, in operation, the evolution of zinc electrodes within three environments, namely alkaline, near-neutral, and mildly acidic. Different combinations of currents were used to demonstrate various scenarios exhibiting both dendritic and smooth deposition of active material. Directly from radiograms, the volume of the electrode and therefore its growth/dissolution rate was estimated and compared against tomographic reconstructions and theoretical values. The protocol combines simplistic cell design with multiple three-dimensional and two-dimensional acquisitions at different magnifications, providing a unique insight into electrode's morphology evolution within various environments.

Electrolyte contact changes nano-Li4Ti5O12 bulk properties via surface polarons.

It is of general interest to combine the faradaic processes based high energy density of a battery with the non-faradaic processes based high power density of a capacitor in one cell. Surface area and functional groups of electrode materials strongly affect these properties. For the anode material Li4Ti5O12 (LTO), we suggest a polaron based mechanism that influences Li ion uptake and mobility. Here we show electrolytes containing a lithium salt induce an observable change in the bulk NMR relaxation properties of LTO nano particles. The longitudinal 7Li NMR relaxation time of bulk LTO can change by almost an order of magnitude and, therefore, reacts very sensitively to the cation and its concentration in the surrounding electrolyte. The reversible effect is largely independent of the used anions and of potential anion decomposition products. It is concluded that lithium salt containing electrolytes increase the mobility of surface polarons. These polarons and additional lithium cations from the electrolyte can now diffuse through the bulk, induce the observed enhanced relaxation rate and enable the non-faradaic process. This picture of a Li+ ion equilibrium between electrolyte and solid may help with improving the charging properties of electrode materials.

Laplace inverted pulsed EPR relaxation to study contact between active material and carbon black in Li-organic battery cathodes.

The addition of conductive additives during electrode fabrication is standard practice to mitigate a low intrinsic electronic conductivity of most cathode materials used in Li-ion batteries. To ensure an optimal conduction pathway, these conductive additives, which generally consist of carbon particles, need to be in good contact with the active compounds. Herein, we demonstrate how a combination of pulsed electron paramagnetic resonance (EPR) relaxometry and inverse Laplace transform (ILT) can be used to study such contact. The investigated system consists of PTMA (poly(2,2,6,6-tetramethylpiperidinyloxy-4-ylmethacrylate)) monomer radicals, which is a commonly used redox unit in organic radical batteries (ORB), mixed at different ratios with Super P carbon black (CB) as the conductive additive. Inversion recovery data were acquired to determine longitudinal (T1) relaxation time constant distributions. It was observed that not only the position and relative amplitude, but also the number of relaxation modes varies as the composition of PTMA monomer and CB is changed, thereby justifying the use of ILT instead of fitting with a predetermined number of components. A hypothesis for the origin of different relaxation modes was devised. It suggests that the electrode composition may locally affect the quality of electronic contact between the active material and carbon black.

Gauging the importance of structural parameters for hyperfine coupling constants in organic radicals.

The identification of fundamental relationships between atomic configuration and electronic structure typically requires experimental empiricism or systematic theoretical studies. Here, we provide an alternative statistical approach to gauge the importance of structure parameters, i.e., bond lengths, bond angles, and dihedral angles, for hyperfine coupling constants in organic radicals. Hyperfine coupling constants describe electron-nuclear interactions defined by the electronic structure and are experimentally measurable, for example, by electron paramagnetic resonance spectroscopy. Importance quantifiers are computed with the machine learning algorithm neighborhood components analysis using molecular dynamics trajectory snapshots. Atomic-electronic structure relationships are visualized in matrices correlating structure parameters with coupling constants of all magnetic nuclei. Qualitatively, the results reproduce common hyperfine coupling models. Tools to use the presented procedure for other radicals/paramagnetic species or other atomic structure-dependent parameters are provided.

Carbonization-Temperature-Dependent Electrical Properties of Carbon Nanofibers-From Nanoscale to Macroscale.

An exact understanding of the conductivity of individual fibers and their networks is crucial to tailor the overall macroscopic properties of polyacrylonitrile (PAN)-based carbon nanofibers (CNFs). Therefore, microelectrical properties of CNF networks and nanoelectrical properties of individual CNFs, carbonized at temperatures from 600 to 1000 °C, are studied by means of conductive atomic force microscopy (C-AFM). At the microscale, the CNF networks show good electrical interconnections enabling a homogeneously distributed current flow. The network's homogeneity is underlined by the strong correlation of macroscopic conductivities, determined by the four-point-method, and microscopic results. Both, microscopic and macroscopic electrical properties, solely depend on the carbonization temperature and the exact resulting fiber structure. Strikingly, nanoscale high-resolution current maps of individual CNFs reveal a large highly resistive surface fraction, representing a clear limitation. Highly resistive surface domains are either attributed to disordered highly resistive carbon structures at the surface or the absence of electron percolation paths in the bulk volume. With increased carbonization temperature, the conductive surface domains grow in size resulting in a higher conductivity. This work contributes to existing microstructural models of CNFs by extending them by electrical properties, especially electron percolation paths.

Effect of Low Environmental Pressure on Sintering Behavior of NASICON-Type Li1.3Al0.3Ti1.7(PO4)3 Solid Electrolytes: An In Situ ESEM Study.

Solid-state sintering at high temperatures is commonly used to densify solid electrolytes. Yet, optimizing phase purity, structure, and grain sizes of solid electrolytes is challenging due to the lack of understanding of relevant processes during sintering. Here, we use an in situ environmental scanning electron microscopy (ESEM) to monitor the sintering behavior of NASICON-type Li1.3Al0.3Ti1.7(PO4)3 (LATP) at low environmental pressures. Our results show that while no major morphological changes are observed at 10-2 Pa and only coarsening is induced at 10 Pa, environmental pressures of 300 and 750 Pa lead to the formation of typically sintered LATP electrolytes. Furthermore, the use of pressure as an additional parameter in sintering allows the grain size and shape of electrolyte particles to be controlled.